It has long been recognized in the prior art that the walls and floor and ceiling of a room form part of the air column for a loudspeaker enclosure and must cooperate therewith. The prior art has provided a number of corner horns which seek this type of cooperation, and which use the two walls and the floor which meet in the corner of the room as an extension of the speaker enclosure air column. Examples of this type of loudspeaker system are included in U.S Pat No. 2,310,243 to Klipsch, in U.S. Pat. No. 2,825,419 to Stephens and in U.S. Pat. No. 2,815,086 to Hartsfield. These speaker enclosures seek to match the expansion beginning in the corner of a room, which expansion can be thought of as extending through one-eighth of a sphere. The ideal speaker enclosure for driving this one-eighth spherical air column would be an enclosure whose horn feeds smoothly thereinto without discontinuities attributable to a sudden change in the expansion rate at the mouth of the horn. The expansion rate known in the prior art is occasionally hyperbolic, but it is usually exponential so that the volume of air in the horn doubles for each unit length thereof commencing at the origin, the rate of expansion having been figured out essentially empirically to provide a family of horns whose cutoff frequency at the lower end of their response curve is determined by the selected expansion rate. These figures are well tabulated, an example of such tabulation appearing in the text "How to Build Speaker Enclosures" by Alexis Badmaieff and Don Davis, Howard W. Sams and Company, Indianapolis, Indiana, 13th printing (1978) at pages 86 and 87.
The present invention relates to a horn system having an expansion rate which becomes catenary between the cabinet and the wall. Although the above publication at page 12 mentions catenary expansion and at page 86 refers to a "Lee Catenary Horn", this is the only reference of which the applicant is presently aware which mentions a catenary expansion for a horn. No further description or specification of the "Lee Catenary Horn" has been found.
One serious disadvantage of a corner horn structure of the type set forth in the three above mentioned patents is that it is essentially unsuitable for stereo reproduction which requires two such cabinets. The corners of a room are generally too far apart for the placement of the two stereo speaker units, which should be placed some six or eight feet apart rather than the full length or width of a room. The corner horn cannot be moved to a more mid-wall location without spoiling its coupling to the air in the room. The present invention seeks to solve this problem by providing a type of cabinet which can be mounted to a mid-wall position so that two such cabinets can be spaced apart on the same wall by the distance required for optimum stereophonic reproduction.
One difficulty encountered in designing a horn for mid-wall location is that it must achieve a much higher expansion rate than a corner horn cabinet because it must drive an air volume in the room which volume is a quarter of a sphere, rather than driving a corner air volume which is one-eighth of a sphere. In attempting to design a horn for mid-wall mounting, it turns out that a cabinet housing a horn which continues an exponential expansion into the room would have to be very large because it would have to fold a long horn many times within the cabinet length. For instance, a horn seeking to maintain an exponential expansion which is good to thirty Hz before cutoff would have to be about sixteen feet long, and this would require many foldings of the horn within the cabinet, these foldings generally being at angles much greater than ninety degrees since the horn usually folds back and forth within the cabinet. Each time that a horn is folded, the fold introduces unpredictable discontinuities since the path length around the fold is quite variable from one side of the horn to the other. This introduces different phase velocities at different frequencies which creates a distortion of the sound coming from the mouth of the speaker. Moreover, folds of the horn exceeding ninety degrees encourage internal reflections of the sound waves inside the horn, thereby causing non-linearities in the performance of the horn at different frequencies. Many of the folded exponential horns have low pass filters located in the vicinity of the speaker for the purpose of restricting the throat area and preventing the mid-range frequencies from entering the horn since these frequencies are more subject to phase distortion than the lower frequencies which the horn is designed to pass with efficiency. Nevertheless, such distortions still result. Moreover, a horn of such length is much more expensive to build and requires an excessively large cabinet, which is undesirable in stereo systems where two cabinets are usually necessary on the same wall.